The present invention relates to rare earth permanent magnet materials, particularly to R-Fe-B permanent magnet materials having good thermal stability.
R-Fe-B permanent magnet materials have been developed as new compositions having higher magnetic properties than R-Co permanent magnet materials (Japanese Patent Laid-Open Nos. 59-46008, 59-64733 and 59-89401, and M. Sagawa et al, "New Material for Permanent Magnets on a Basis of Nd and Fe," J. Appl. Phys. 55 (6) 2083(1984)). According to these references, an alloy of Nd.sub.15 Fe.sub.77 B.sub.8 [Nd(Fe.sub.0.91 B.sub.0.09).sub.5.67 ], for instance, has such magnetic properties as (BH)max of nearly 35 MGOe and iHc of nearly 10 KOe. The R-Fe-B magnets, however, have low Curie temperatures, so that they are poor in thermal stability. To solve these problems, attempts were made to elevate Curie temperature by adding Co (Japanese Patent Laid-Open No. 59-64733). Specifically, the R-Fe-B permanent magnet has Curie temperature of about 300.degree. C. and at highest 370.degree. C. (Japanese Patent Laid-Open No. 59-46008), while the substitution of Co for part of Fe in the R-Fe-B magnet serves to increase the Curie temperature to 400-800.degree. C. (Japanese Patent Laid-Open No. 59-64733). However the addition of Co decreases the coercive force iHc of the R-Fe-B magnet.
Attempts were also made to improve the coercive force by adding Al, Ti, V, Cr, Mn, Zn, Hf, Nb, Ta Mo, Ge, Sb, Sn, Bi, Ni, etc. It was pointed out that Al is particularly effective to improve the coercive force (Japanese Patent Laid-Open No. 59-89401). However, since these elements are non-magnetic except for Ni, the addition of larger amounts of such elements would result in the decrease in residual magnetic flux density Br, which in turn leads to the decrease in (BH)max.
Further, the substitution of heavy rare earth elements such as Tb, Dy and Ho for part of Nd was proposed to improve coercive force while retaining high (BH)max (Japanese Patent Laid-Open Nos. 60-32306 and 60-34005). By substituting the heavy rare earth element for part of Nd, the coercive force is enhanced from 9 KOe or so to 12-18 KOe for (BH)max of about 30 MGOe. However, since heavy rare earth elements are very expensive, the substitution of such heavy rare earth elements for part of neodymium in large amounts undesirably increases the costs of the R-Fe-B magnets.
In addition, the addition of both Co and Al was proposed to improve thermal stability of the R-Fe-B magnet (T. Mizoguchi et al., Appl. Phys. Lett. 48. 1309 (1986)). The substitution of Co for part of Fe increases Curie temperature Tc, but it acts to lower iHc, presumably because there appear ferromagnetic precipitation phases of Nd (Fe, Co).sub.2 on the grain boundaries, which form nucleation sites of reverse domains. The addition of Al in combination with Co serves to form non-magnetic Nd(Fe,Co,Al).sub.2 phases which suppress the generation of the nucleation sites of reverse magnetic domains. However, since the addition of Al greatly decreases Curie temperature Tc, R-Fe-B magnets containing Co and Al inevitably have poor thermal stability at as high temperatures as 100.degree. C. or more. In addition, the coercive force iHc of such magnets is merely 9 KOe or so.